Recent efforts to improve the fuel economy and emissions levels of air and ground vehicles have created a need for new materials which can provide weight savings to the vehicle without sacrificing performance levels. The immediate desirability of such materials is enhanced when the weight savings can be acheived by directly substituting the materials for current materials in existing designs. Moreover, the long-term desirability of such materials is maximized when the unique properties of the materials provide the possibility of improved designs and performance for vehicle components.
Traditionally, automotive brake rotors have been made from cast iron which provides good wear resistance and excellent high temperature properties. However, cast iron is dense relative to other candidate materials and, therefore, a cast iron brake rotor is relatively heavy. A heavy brake rotor is considered to be undesirable for at least three reasons. The first reason is that a heavy brake rotor contributes to the overall weight of the vehicle and thus reduces its fuel efficiency and correspondingly increases its emissions levels. The second reason (relevant mainly to passenger cars and trucks) is that a brake rotor is part of the "unsprung" weight of a vehicle (i.e., the weight of a vehicle that is below the springs) and, as such, contributes to the noise, vibration and harshness (commonly known in the automobile industry as "NVH") associated with the operation of the vehicle. When the unsprung weight of a vehicle is reduced, the NVH properties are usually improved. The third reason is that a brake rotor is a part of a vehicle that requires rotation during use and, accordingly, a heavier brake rotor requires the use of additional energy to increase and decrease the rotational speed of the rotor. In addition, the ability of a heavier brake rotor to cause undesirable vibration during rotation is greater than that associated with a lighter brake rotor.
The search for a material to replace cast iron in brake rotors has identified several possible candidates and their advantages and limitations. Each of these materials and its relevant advantages and limitations is discussed below.
Steel has been considered as a brake rotor material because of its excellent strength to weight properties. Although denser than cast iron, the superior strength of steel enables the use of smaller brake rotors which could result in weight savings. However, at the present time, the weight savings that have been obtained with steel brake rotors have been minimal.
Titanium has also been considered as a brake rotor material. The excellent strength to weight properties of titanium, as well as its high temperature properties, would enable titanium brake rotors to satisfy all of the requirements discussed above for a desirable brake rotor material. However, the high cost of titanium has prevented its widespread use as a brake rotor material in most non-aerospace applications.
Various polymeric materials have also been considered as brake rotor materials. These materials have the advantage of being relatively inexpensive but they have not been able to achieve the high temperature strength necessary to perform adequately as a brake rotor material.
Various ceramic materials have also been considered as brake rotor materials. Although many ceramic materials have demonstrated excellent wear resistance properties and the ability to withstand extremely high temperatures, the brittle nature of most ceramic materials has precluded the widespread use of ceramic brake rotors. Although the use of new processing techniques and the inclusion of reinforcing materials has created a new generation of ceramic and ceramic matrix composite materials with increased strength and reduced brittleness that perform well as brake rotor materials, the present production cost of such materials relative to other available materials has not been able to justify, for most ground vehicles, the weight savings that many of these materials can provide relative to cast iron. However, some of these new ceramic and ceramic matrix composite materials are being tested for use as brake rotor materials in heavier ground vehicles and/or in vehicles that demand increased performance from their brake rotors. In these situations, the higher cost of such materials is justified by their ability to provide increased performance.
Aluminum and magnesium alloys have also been considered as brake rotor materials. These metals show excellent strength to weight properties but their high temperature properties are not adequate for most brake rotor applications. Specifically, brake rotor tests using both magnesium and aluminum rotors have demonstrated that unacceptable amounts of surface scoring and rotor warpage occur after repeated braking cycles. These problems can be partially alleviated by incorporating various alloying elements into the magnesium and aluminum metals and/or heat treating the final brake rotors before use. However, the use of such additives and/or techniques raises the cost of the brake rotors and can cause the rotors to display undesirable side effects, such as increased brittleness and high temperature instability. Accordingly, the use of alloying additives and heat treatment techniques, either alone or in combination, has not been able to produce commercially viable brake rotors for most of the current brake rotor applications.
Recent attempts to reduce or eliminate the problems associated with using aluminum and magnesium as brake rotor materials have been directed toward the production of various types of aluminum and magnesium metal matrix composite materials. These materials generally consist of a metal matrix having embedded therein one or more reinforcing materials. Several techniques for forming metal matrix composites have been developed, some of which use pressure or a vacuum to push or draw a molten metal into a mass or preform of reinforcing material (hereinafter sometimes referred to as "filler material" or "filler"). Other techniques for forming metal matrix composite materials do not require the use of pressure or a vacuum to enable the molten metal to infiltrate the filler material. Such infiltration techniques are sometimes referred to as "spontaneous infiltration" techniques. Representative methods for forming metal matrix composites and/or casting metals can be found in the following Patents:
U.S. Pat. No. 5,028,392, which issued on Jul. 2, 1991, in the names of Lloyd et al., and entitled "Melt Process For the Production of Metal-Matrix Composite Materials With Enhanced Particle/Matrix Wetting"; PA1 U.S. Pat. No. 5,028,494, which issued on Jul. 2, 1991, in the names of Tsujimura et al., and entitled "Brake Disk Material For Railroad Vehicle"; PA1 U.S. Pat. No. 4,865,806, which issued on Sep. 12, 1989, in the names of Skibo et al., and entitled "Process For Preparation of Composite Materials Containing Nonmetallic Particles In A Metallic Matrix"; PA1 U.S. Pat. No. 4,759,995, which issued on Jul. 26, 1988, in the names of Skibo et al., and entitled "Process For Production of Metal Matrix Composites By Casting and Composite Therefrom"; PA1 U.S. Pat. No. 4,961,461, which issued on Oct. 9, 1990, in the names of Klier et al., and entitled "Method and Apparatus For Continuous Casting of Composites"; PA1 U.S. Pat. No. 4,473,103, which issued on Sep. 25, 1984, in the names of Kenney et al., and entitled "Continuous Production of Metal Alloy Composites"; PA1 U.S. Pat. No. 4,404,262, which issued on Sep. 13, 1983, in the name of Watmough, and entitled "Composite Metallic and Refractory Article and Method of Manufacturing the Article"; PA1 U.S. Pat. No. 3,970,136, which issued on Jul. 20, 1976, in the names of Cannell et al., and entitled "Method of Manufacturing Composite Materials"; PA1 U.S. Pat. No. 3,915,699, which issued on Oct. 28, 1975, in the names of Umehara et al., and entitled "Method For Producing Metal Dies or Molds Containing Cooling Channels By Sintering Powdered Metals"; PA1 U.S. Pat. No. 3,718,441, which issued on Feb. 27, 1973, in the name of Landingham, and entitled "Method For Forming Metal-Filled Ceramics of Near Theoretical Density"; PA1 U.S. Pat. No. 5,042,561, which issued on Aug. 27, 1991, in the name of Chandley and entitled "Apparatus and Process for Countergravity Casting of Metal With Air Exclusion"; PA1 U.S. Pat. No. 4,862,945, which issued on Sep. 5, 1989, in the names of Greanias et al. and entitled "Vacuum Countergravity Casting Apparatus and Method With Backflow Valve"; PA1 U.S. Pat. No. 3,547,180, which issued on Dec. 15, 1970, in the name of Cochran, and entitled "Production of Reinforced Composites"; and PA1 U.S. Pat. No. 3,364,976, which issued on Jan. 23, 1968, in the names of Reding et al., and entitled "Method of Casting Employing Self-Generated Vacuum".
The entire disclosures of all of the above-listed U.S. Patents are expressly incorporated herein by reference.
An example of a metal matrix composite brake rotor can be found in U.S. Pat. No. 5,028,494, which issued on Jul. 2, 1991, in the names of Tsujimura et al. (hereinafter referred to as the '494 Patent). In the '494 Patent, an aluminum composite material is produced as a brake disk material for railroad vehicles. In the method of the '494 Patent, reinforcement particles of alumina, silicon carbide, mica or the like are dispersed and mixed into a molten aluminum alloy. The reinforcement particles are 5 to 100 microns in diameter, and are dispersed uniformly in the alloy in an amount of 1 to 25% by weight (i.e., about 0.7% to about 18.4% by volume for alumina reinforcement material; about 0.8% to about 22.0% by volume for silicon carbide reinforcement material and about 1.0% to about 25.7% by volume for mica reinforcement material). It is stated in the '494 Patent that the brake disk material produced by the method disclosed in the '494 Patent is "light in weight and has high strength, good thermal conductivity and high wear resistance."
Thus, it can be deduced from the above information that metal matrix composite materials are currently being examined and tested for use as brake rotor materials. Moreover, it should be noted that the metal matrix composite brake rotors currently being produced for the railroad vehicle industry (as evidenced by the '494 Patent) use an aluminum metal matrix with a reinforcement material loading of up to about 26% by volume.
It has been unexpectedly discovered that brake rotors produced from metal matrix composites having reinforcement loadings of at least about 26% by volume, and preferably at least about 28% by volume, demonstrate unexpectedly enhanced performance in comparison to materials with lower reinforcement loadings (i.e., reinforcement loadings lower than about 26% by volume). Specifically, many metal matrix composite brake rotors produced with less than about 26% by volume of reinforcement material have been unable to meet industry performance requirements in certain tests known as "fade tests" (discussed in detail later herein) wherein the brake rotor is repeatedly tested under cyclical braking conditions (e.g., the brake rotor is mounted on a vehicle braking system or a dynamometer and used to brake a vehicle from about 60 mph to 0 mph several times and then from about 80 mph to 0 mph for a required number of times or until failure). Such brake rotors exhibit unacceptable surface scoring (i.e.,surface disfigurements, such as scratches or grooves) after the fade tests and, in some cases, portions of the brake rotors (e.g., the cooling fins and/or the rotor surface which contacts the brake pad) were deformed and appeared to have been melted during the tests.
In contrast, metal matrix composite brake rotors having reinforcement loadings greater than about 26% by volume, and preferably greater than about 28% by volume, have easily survived the above-described fade tests with acceptable levels of surface scoring and no significant deformation. Further, it has been determined that the ability of a rotor to withstand certain standard industry tests which simulate some of the most severe conditions experienced by automotive rotors can be discussed in terms of the maximum operating temperature ("MOT", discussed in detail later herein) which a rotor can withstand prior to experiencing at least some undesirable surface melting. The rotors of the present invention exhibit an MOT of at least about 900.degree. F. (482.degree. C.), and preferably at least about 950.degree. F. (510.degree. C.) and even more preferably at least about 975.degree. F. (524.degree. C.), and, even more preferably, about 1000.degree. F. (538.degree. C.) and higher.
Moreover, it has been discovered that certain ceramic matrix composites can also achieve the aforementioned MOT's and higher. Certain preferred techniques for forming ceramic matrix composites are discussed herein.
Accordingly, the increasing demand for higher fuel efficiency and reduced emissions has created a need for brake rotors on ground vehicles that are capable of satisfying current performance requirements while providing weight savings to the overall vehicle with respect to the brake rotors currently in use. The present invention provides brake rotors that can satisfy these needs.